The present invention relates to a gate drive circuit for a voltage-driven switching device, such as an insulated gate bipolar transistor (IGBT).
FIG. 6 shows a conventional example of a chopper circuit comprising such a voltage-driven switching device and a gate drive circuit.
A switching circuit 1 composed of an IGBT 1a and a diode 1b connected inversely in parallel thereto is connected in series to a switching circuit 2 composed of an IGBT 2a and a diode 2b connected inversely in parallel thereto, and both ends of a combination of these circuits are connected to a DC power supply 9 via a wiring inductance 11. In addition, a load inductance 10 is connected to both ends of the switching circuit 2, and a capacitor 12 is connected as a snubber circuit to both ends of a series circuit composed of the switching circuits 1 and 2. In addition, a gate drive circuit 3 is connected to the IGBT 1a. The gate drive circuit 3 includes DC power supplies 7a and 7b connected in series; a resistor 8a connected at one end to a positive electrode side of the DC power supply 7a; a switch 6a connected at one end to the other end of the resistor 8a; a switch 6b connected at one end to the other end of the switch 6a; and a resistor 8b connected at one end to the other end of the switch 6b, and to a negative electrode side of the DC current 7b at the other end thereof. A gate terminal of the IGBT 1a is connected to the connection between the switches 6a and 6b, and an emitter terminal of the IGBT 1a is connected to the connection between the DC power supplies 7a and 7b.
The chopper circuit shown in FIG. 6 turns the IGBT 1a on and off to adjust an output power. This operation is illustrated in FIG. 7.
First, the switch 6a in the gate drive circuit 3 is closed, while the switch 6b is opened. A current depending on the DC power supply 7a, the resistor 8a and the input capacitance of the IGBT la is allowed to flow through the gate terminal of the IGBT 1a as a main switch to provide an electric charge between the gate and emitter in order to turn on the IGBT 1a. Once the IGBT la has been turned on, a current io (see FIG. 6) flows through its output. Then, the switch 6a of the gate drive circuit 3 is opened while the switch 6b is closed to discharge the electric charge between the gate and emitter of the IGBT 1a, thereby turning off the IGBT 1a. This causes the current flowing through the load inductance 10 to flow through the diode 2b for circulation. This current is called as a free wheeling current. In this manner, the power supplied to the load can be adjusted by repeatedly turning the main switch on and off.
Such a chopper circuit has the following problems:
Since the diode 2b is on-state by the free wheeling current flowing through the load inductance 10 immediately after the IGBT 1a has been turned on, a current path is formed between the IGBT 1a and the diode 2b, and the current through the diode 2b rapidly decreases when a current increase ratio di/dt of the IGBT 1a is large. The diode 2b then starts a reverse recovery to interrupt the current, so the wiring inductance 11 causes a surge voltage of its inductance 11 multiplied by the current increase ratio di/dt plus an intermediate DC voltage Ed, such as one shown in FIG. 7, which is applied to the diode 2b. Required are devices that can withstand this surge voltage. However, the devices with high withstanding voltage increase costs and result in an increased forward voltage drop, to thereby increase steady loss and reduce conversion efficiency.
Alternatively, to absorb the surge voltage, a snubber circuit 12 such as one shown in FIG. 6 may be installed parallel to the series circuit of the switching circuits. But this method decreases conversion efficiency caused by loss to the snubber circuit and increases both apparatus size and cost, due to installation of the snubber circuit.
Alternatively, to inhibit the surge voltage, a gate resistance may be increased to reduce the voltage decrease ratio dv/dt of the IGBT. But this method reduces a switching speed to increase the switching loss. The increase in the switching loss necessitates a larger cooling fan, leading to a bulkier and more expensive apparatus.
The object of the invention is to reduce costs without increasing apparatus size or reducing conversion efficiency.